Prefetch 1.0

Hi Brian,

I have tagged my recent development before improving prefetch to queue IO read request.

Added cache statistics:
 - keep cache statics and log it in IO::Detach()
 - add  test/parseRec.pl which parses statistics from xrootd log file

Improve logging:
- introduce debug levels and optimise print with help of variadic macros defined in Context.hh
- use formatted print instead of stringstream